1. Field of the Invention
The present invention relates to a method and system for evaluating the condition of a medium and, more particularly, relates to a method and system for approximating the values of unstable physical conditions of a medium.
2. Prior Art
Determination of physical conditions of a medium frequently requires exposure of a condition sensor to the medium in such a way that the sensor indicates a changing condition level until a sensed condition "dwell" occurs where the sensor indication tends to remain constant. The dwelling sensor indication value represents the condition of the medium. Conditions of the character referred to are, for example, pressure, temperature, conductivity, or some other condition which bears a known relationship to the measured condition.
Sensors which produce a continuously variable, or analog, sensed condition signal have been commonly used for sensing temperature and pressure levels. The analog condition signals have often controlled operation of pen recorders or similar signal display and recording devices so that the instantaneous level of the sensed condition at any particular time is graphically displayed and determinable. By viewing the graphic representation of the condition as sensed, an individual having experience with the medium and the condition being sensed can form fairly reliable judgments concerning actual condition levels of the medium.
By way of example, when the temperature of a constant temperature liquid bath is to be determined, a thermocouple or other suitable sensor device is immersed in the bath and heat transfer to or from the bath changes the thermocouple temperature until the thermocouple and bath are in equilibrium. The analog electrical output signal from the thermocouple will vary substantially from moment to moment until the thermocouple temperature approaches the bath temperature, at which time the rate of change of the signal is reduced. When the thermocouple is in equilibrium with the bath, the signal ceases changing and a temperature dwell is encountered.
Instruments for automatically determining temperature in such circumstances have been proposed. These instruments have generally employed electronic circuitry for determining the instantaneous rate at which the sensed temperature signal changes. When the rate of the signal change is zero or close to zero (indicating a dwell), the circuitry may operate to produce a quantitized output signal indicating the dwell temperature level.
In some circumstances, the existence of a condition dwell and the level of the dwell are difficult to determine because the medium condition is unstable. Unstable temperature conditions tend to be encountered in high temperature liquid or gases which frequently contain zones of differing temperatures. The temperature of molten steel in a basic oxygen furnace, for example, cannot be precisely determined primarily because the steel contains circulating convection currents and thus temperatures vary with time and location in the furnace.
A typical analog sensed temperature trace for such steel is characterized by a series of peaks and valleys which may not clearly indicate a sensed temperature dwell or a particular dwell level. Prior art circuitry for recognizing dwells and producing output dwell temperature readings are not suitable for use because the instantaneous rate of change of the sensed temperature with respect to time does not necessarily indicate a dwell when it reaches zero. Accordingly, automatic condition determination using the prior art teachings has not been universally possible.
Steel-making is one field where automatic condition determination has not been used. In basic oxygen furnace operation, for example, skilled attendants have traditionally monitored time-temperature traces produced by pen recorders for the purpose of judging when the trace has "dwelled" and for approximating the steel temperature. This process has been, at least to some extent, subjective and therefore not necessarily consistent from time to time and attendant to attendant.
The same has been true for oxygen content measurements of molten steel. Oxygen content of steel is commonly measured by a cell capable of producing an electrical signal which varies as a function of sensed partial pressure of oxygen. Oxygen partial pressure tends to vary and is thus unstable in molten steel. Furthermore the amount of dissolved oxygen in the steel depends on the steel temperature. When either, or both, the oxygen and temperature conditions are not properly, or accurately, evaluated, serious problems are encountered later on in the steel-making process.
For the reasons indicated, prior art proposals for evaluating unstable sensed conditions have not been completely satisfactory, particularly in iron and steel production.